In compression molding operations, efficient filling of the molds with materials in the powder form has a significant bearing in determining the unit manufacturing cost (UMC). Feeder boxes arc used to carry loose powder to the top of the die where it shakes dropping powder into the die cavity. The die cavity is located underneath the die clamp ring or the die retaining ring. The die cavity is filled by the shaking motion of the feeder box which is connected to a hydraulic actuated cylinder. The feeder box slides on a flat surface called "ware plate" in a reciprocating motion. The time interval between the two extreme locations of the feeder box, when the compacting of the powders in the die cavity is effected. The top surfaces of the ware plate, the die retaining ring and the bottom surface of the feeder box need to be at the same level for the smooth operation of the powder filling mechanism. When the die cavity is filled with powder, it is compressed by the action of punches making a "green part". The proper filling of the die cavity depends, among several other factors, on the sliding motion of the bottom of the feeder box on a plate normally known as feeder box ware plate and also on the die clamp (or retaining) ring. If there is a misalignment of the feeder box with respect to the ware plate and also with respect to the die clamp (or retaining) ring or if there is a slight gap between those components, excessive powder loss results. This problem is magnified if the powder particles are very fine (submicron) or are very hard. In such cases, the jamming of the feeder box (in worst case, breakage of the feeder box or ware plate or die clamp ring) can lead to interruption of the manufacturing process and a higher UMC for the part.
Misalignment of the feeder box with respect to the ware plate and/or with respect to die clamp ring can occur due to gouging of one of the surfaces, which is a common occurrence in industrial compression molding machines where the box and the plate and also the clamp ring are usually made of steels. Repeated sliding of two surfaces of metal parts, as in this specific case of surfaces of feeder box and the ware plate (or die clamp ring), usually leads to excessive wear and abrasion of those surfaces leading to the loss of materials, contaminating the powder feed, and also creating a gap between the box and the plate and also between the feeder box and the die clamp ring. This gap between the feeder box and the ware plate and also between the feeder box and the die clamp ring leads to the loss of powder, and in some cases jamming of the feeder mechanism. Normally, the machine components will begin to show signs of wear at about 5000 cycles of operation. At about one million cycle, the feeder plate will have to be replaced, if not sooner. Powdered material will start leaking out from under the feeder box and falling on the rest of the movable components of the machine making maintenance a large problem. Also, the motion of the feeder box will become agitated and impede the free flowing motion of powder during die filling.
In order to overcome the aforementioned problems one needs to search for the right material. Experience indicates that ytrria-doped tetragonal zirconia polycrystal (Y-TZP) ceramic materials offer many advantages over conventional materials, including many other ceramics. Y-TZP is one of the toughest ceramics. The toughness is achieved at the expense of hardness and strength. Tetragonal zirconia alloy-alumina composite, that is, the product of sintering a particulate mixture of zirconia alloy and alumina, is another tough and relatively soft structural ceramic composite. Y-TZP ceramic and zirconia-alumina composites have tribological properties that are not as attractive as other high performance structural ceramics like SiC and Si.sub.3 N.sub.4. However, most of the conventional high performance ceramics are extremely brittle. An example of a material having good hardness and strength is monolithic cubic spinel, however, this material is also highly brittle and is unusable for structural applications.
It is further known that a powder feed assemblage having a feeder box surface, particularly the lower surface of the box in sliding contact with a ware plate surface and die clamp ring has a longer service life and better performance if made with a relatively hard material having high fracture toughness and the mating surfaces have low coefficient friction.
An alternative approach is taught by U.S. Pat. No. 5,358,913, which is hereby incorporated herein by reference. In that approach, a tetragonal zirconia alloy article, which can be near net-shape, is compacted and then sintered in the presence of an MgO, CaO, Y.sub.2 O.sub.3, Sc.sub.2 O.sub.3, Ce.sub.2 O.sub.3, or other rare earth oxide dopant to produce an article having a tetragonal core and a cubic case. The dopant can be provided in a number of different forms such as a solid plate, a powder, or a layer produced by decomposition of an organo-metallic precursor film. In U.S. patent application Ser. No. 07/994,820, now abandoned in favor of Continuation-in-Part application Ser. No. 08/231,870, filed Apr. 25, 1994, a method is described for producing articles having a tetragonal zirconia alloy core and a monoclinic case. In U.S. patent application Ser. No. 07/994,818, now abandoned in favor of a Continuation-in-Part application U.S. Ser. No. 08/400,416, hereby incorporated by reference, a method is described for producing articles having a tetragonal zirconia alloy and .alpha.-alumina (alpha-Al.sub.2 O.sub.3) core and a case of tetragonal zirconia and cubic spinel. In the core and the case the predominant species is tetragonal zirconia. The application also teaches a method for producing articles having a core that is tetragonal zirconia alloy along with less than about 5 weight percent alumina and having a case that is cubic phase zirconia and cubic spinel (MgAl.sub.2 O.sub.4). a-alumina is about as hard as cubic zirconia. These types of ceramics with composite structures with varying degrees of physical and mechanical properties are termed as "functionally gradient ceramics". In functionally gradient ceramics, where the core and shell of the ceramic bodies having different crystal structures hereby disclosed in above-referenced U.S. Patents and U.S. Patent Applications.
As will be more completely disclosed, the method of our invention applies to a powder feeder assemblage, i.e., a complete set of stationary ware plate, die clamp ring and sliding feeder box bottom surface made of ceramic and ceramic composites, particularly one type member of the assemblage is made of Y-TZP. If the stationary parts are made of Y-TZP, the sliding part is made of either zirconia-alumina composites or of functionally gradient ceramics based on Y-TZP ceramic or ZrO.sub.2 --Al.sub.2 O.sub.3 composites. Proper choices of ceramics in manufacturing these feeder box, die clamp ring and ware plate assembly are essential to overcome the problems described above.
Therefore, a need persists for an apparatus and method of making precision ceramic powder feed assemblies that have superior wear and abrasion resistance while being cost effective and easy to manufacture.